Focus peaking good for long tele?

I use an A*300mm f4 for shooting wildlife and sports, mostly used wide open, and often with a Kenko 1.5X TC. I have trouble nailing the focus with my K20D. Would focus peaking on a K-30 raise my hit rate? Is there any downside to focus peaking vs regular manual focus? What about speed, does focus peaking help to acquire focus faster?

I also have a K-x with Pentax O-ME53 magnifier, which I only use when I'm stuck with high ISO shooting conditions. I'd debating whether to go for a body upgrade, or get the DA*300mm. I'm hesitating on the lens upgrade because I often shoot birds in trees where AF doesn't help, and I don't believe my Kenko PZ TC will be useable with the DA*300.
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Dan